In a vehicle, a differential is part of a power train. The differential employs gears that are driven by an input drive shaft. The gears are connected to two output drive shafts. In this manner, torque and rotation are transmitted from an engine to wheels.
Typically, a drive ring gear assembly, which comprises a matched set of a drive ring gear and pinion gear, is attached to a differential carrier. The drive ring gear is driven by the pinion gear, which is connected to the input drive shaft. The drive ring gear mechanically passes the input torque and rotation from the pinion gear onto the wheels. The drive ring gear may be a hypoid gear type having spiral bevel teeth that are helical in shape. The helical design of the hypoid shaped drive ring gear produces less vibration and noise than conventional straight-cut or spur-cut gears with straight teeth.
In the past, the drive ring gear had been connected to the differential case assembly by way of many special large drive bolts utilizing loose fit clearance holes in the differential case and tapped holes in the drive ring gear. In this regard, it has been found that only two of the special large drive bolts may actually be in contact with the loose fit clearance differential case holes, thereby only two of the bolts are actually carrying the full operational torsional load.
Other means of connecting the drive ring gear to the differential case assembly are by way of standard welding or brazing. More recently, the drive ring gear is connected to the differential case assembly by way of laser welding.
Specifically, laser welding is known for high power density (on the order of 1 MW/cm2) which results in small heat-affected zones with high heating and subsequently rapid cooling rates. The spot size of such laser welds can vary between 0.2 mm and 13 mm, though only the smaller sizes are typically used for welding. The depth of penetration of the laser welds is proportional to the amount of power supplied, but is also dependent on the location of the focal point; penetration being maximized when the focal point is slightly below the surface of the work piece.
A continuous or pulsed laser beam may be used, depending upon the application. Milliseconds long laser beam pulses are used to weld thin materials, while continuous laser beams are employed for deep welds, like those required for a thick drive ring gear.
As a result, the quality of laser welds themselves is high, where the speed of welding is proportional to the amount of power supplied, which highly depends on the type and thickness of the work pieces. The high power capability of gas lasers, however, makes them especially suitable for high volume applications. Consequently, laser welding is particularly dominant for automotive parts, where carbon steels, high-strength low-alloy (HSLA) steels, stainless steel, aluminum, and titanium may be laser welded.
With increased usage of laser welding techniques to attach the drive ring gear to the differential case assembly, the many special drive bolts of the past have been eliminated. However, due to the high weld temperatures and rapid cooling rates associated with laser welding, the drive ring gear can become distorted. For example, the back face of the drive ring gear can become concave as it cools following laser welding of the drive ring gear to the differential case assembly.
Unfortunately, distortion from laser welding is not truly correctable since the hypoid tooth form of the drive ring gear is typically not ground, but instead is finished machined and lapped only. As a result, the tightly held dimensions expected of the gear teeth are distorted, which causes the vehicle differential to not operate properly and/or may result in noisy operation.
Consequently, it would be beneficial to utilize a means, other than laser welding, to attach the drive ring gear to the differential case assembly. This means must not distort the dimensions of the gear teeth and should not require the use of expensive drive bolts (which do not distribute the operational torsional load equally), tapped holes, and associated additional labor costs.